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Don't take things so literally. himself, herself, ...what if you don't know the sex...itself. The rest of your question might be relevant to mathematics.SE.
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MitchFeb 25 '13 at 1:27

What @Mitch says. If you try and puzzle that out you'll be at it 'til the cows come home.
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JimFeb 25 '13 at 2:04

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Why can't I add a number to itself? Sure, I can't do that with apples, but apples aren't numbers to begin with.
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jwodderFeb 25 '13 at 5:04

1 Answer
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"Itself" in these expressions simply means "the same number" or "a number of equal value". It's just a lexical shortcut that everyone understands, not an existential problem. Every "1" is equal to every other "1"; they're fungible, like clones. It's a standard idiom in math class, so, as Mitch says, don't be so literal when interpreting the expression. It has a recognized meaning in context: "Add 1 to itself" = "Add 1 and 1".